Sunday, November 18, 2007

The accompanying chart has been marked to delineate landable airports and the altitudes need to reach them for aerotowing a Grob 103 to Truckee. Rich Miller wrote the computer program to plot the data using MatLab to draw the "chain-like" lines surrounding each airport that he selected as a landing option and to calculate the altitudes. The airports themselves are specified by a number next to them in hundreds of feet that indicate the arrival altitude for that airport – pattern altitude plus 1000 feet. For example, Truckee is indicated with "79". The airport is 59 hundred feet ASL plus 10 hundred feet for patterns, plus 10 hundred feet for checklist.

He computed the boundaries surrounding each selected airport – sectors – using a glide slope of 21:1 (3.5 nautical miles per thousand feet). You’ll notice two things about these sectors. First the boundaries are only straight lines when the airports are at the same altitude – Lodi and Franklin for example. Second the airports can be significantly closer to one edge of the sector when the altitude difference between adjoining airports is significant – Placerville and Georgetown for example. No corrections has been made for terrain altitude or wind.

Flying a direct route between Byron and Truckee takes you over Hangtown and Squaw VOR’s. The maximum altitude you’ll need is a little over 12,200 feet as you transition from a potential landout at Georgetown to final glide to Truckee. Blue Canyon is not a landout option on this route.

In the Central Valley there are multiple airports within each sector except for Byron and Funny Farm. Making each airport a separate sector would only clutter the chart.

The absolute furthest distances you can get from a landout airport are at the corners of the sectors. Flying above these altitudes adds an extra margin of safety. In the Central Valley the "maximum-minimum" safe altitude is about 5400 feet near Linden VOR.

Flying an indirect route through Donner Pass saves about 800 feet in altitude gain. This route makes Blue Canyon a landout option as you transition out of the Georgetown sector. The minimum safe altitude to reach Truckee from Soda Springs, or return to Blue Canyon is 11,400 feet (114 on the chart). Sierraville is an equal glideable distance from Soda Springs as well.

The boundaries between sectors are curved because they are hyperbolas (a straight line being a degenerate form of a hyperbola). Two airports are equally glideable along a curve, the points on which are closer to the higher airport by a distance that is equal to the distance the glider can fly in the altitude difference between the airports. This is a definition of a hyperbola with the airports the foci and the difference in distance to the two foci the distance the glider can glide in the elevation difference between the two airports.

The chart is for educational purposes and not to be used for navigation. This work was not supported by any government grant nor were any animals injured or killed as a result of this research .

About the Buzzard

The Buzzard is the online journal of the Northern California Soaring Association(NCSA), a soaring club located in the San Francisco Bay area. The club operates generally on weekends, and flies out of Byron Airport.